Viscoelastic wellbore treatment fluid

ABSTRACT

A viscoelastic wellbore treatment fluid comprises an effective amount of an oligomeric surfactant for controlling the viscoelasticity of the fluid.

FIELD OF THE INVENTION

The present invention relates to viscoelastic wellbore treatment fluids,and particularly treatment fluids comprising oligomeric surfactants.

BACKGROUND OF THE INVENTION

In the recovery of hydrocarbons, such as oil and gas, from naturalhydrocarbon reservoirs, extensive use is made of wellbore treatmentfluids such as drilling fluids, completion fluids, work over fluids,packer fluids, fracturing fluids, conformance or permeability controlfluids and the like.

In many cases significant components of wellbore fluids are thickeningagents, usually based on polymers or viscoelastic surfactants, whichserve to control the viscosity of the fluids. Typical viscoelasticsurfactants are N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammoniumchloride and potassium oleate, solutions of which form gels when mixedwith corresponding activators such as sodium salicylate and potassiumchloride.

Conventional surfactant molecules are characterized by having one longhydrocarbon chain per surfactant headgroup. In the viscoelastic gelledstate these molecules aggregate into worm-like micelles. Gel breakdownoccurs rapidly when the fluid contacts hydrocarbons which cause themicelles to change structure or disband.

In practical terms the surfactants act as reversible thickening agentsso that, on placement in subterranean reservoir formations, theviscosity of a wellbore fluid containing such a surfactant variessignificantly between water- or hydrocarbon-bearing zones of theformations. In this way the fluid is able preferentially to penetratehydrocarbon-bearing zones.

The application of viscoelastic surfactants in both non-foamed andfoamed fluids used for fracturing subterranean formations has beendescribed in several patents, e.g. EP-A-0835983, U.S. Pat. Nos.5,258,137, 5,551,516, 5,964,295 and 5,979,557.

The use of viscoelastic surfactants for water shut off treatments andfor selective acidizing is discussed in GB-A-2332224 and Chang F. F.,Love T., Affeld C. J., Blevins J. B., Thomas R. L. and Fu D. K., “Casestudy of a novel acid diversion technique in carbonate reservoirs”,Society of Petroleum Engineers, 56529, (1999).

A problem associated with the use of viscoelastic surfactants is thatstable oil-in-water emulsions can be formed between the low viscositysurfactant solution (i.e. broken gel) and the reservoir hydrocarbons. Asa consequence, a clean separation of the two phases can be difficult toachieve, complicating clean up of wellbore fluids. A factor promotingemulsion formation is believed to be a reduction of the oil/waterinterfacial energy caused by a tendency for the surfactant molecules tocollect at the water/oil interface.

The recovery of hydrocarbons, such as oil and gas, from a subterraneanwell formation can be impeded by scales obstructing the flow ofhydrocarbons from hydrocarbon-bearing zones of the formation. Typicalscales are barite (BaSO₄) or calcite (CaCO₃) and it is common practiceto treat these by bull-heading an aqueous-based scale dissolver fluidthrough a well bore and into the formation.

For example, one conventional scale dissolver for barite scale consistsof a concentrated solution of potassium carbonate, potassium hydroxideand the potassium salt of ethylenediaminetetraacetic acid (EDTA), thecorrosive and chelating nature of the solution being effective inremoving scale. Carbonate scales may be dissolved using simple mineralacids, such as HCl.

However, hydrocarbon-producing wells often contain zones that arewatered-out, producing only, or very largely, water. If the scaledissolver enters these zones, scale may also be removed therefrom. Thiscan lead to an undesirable increase in the water cut of the fluidproduced by the well.

Dimer surfactants have found some application in fluids used in theexploration and production of hydrocarbons. B. A. M. Oude Alink, “Fattyacids in oil field chemicals” in Fatty Acids in Industry, eds. R. W.Johnson and E. Fritz, pp. 407-429, Marcel Dekker, New York, (1989) andHenkel Corporation Chemicals Group, Abstracts of Dimer Acid Use-Patentsand Journal References, Vol. 1, Technical Bulletin 109A, 1968 review theuse of dimer oleic acids in the production of corrosion inhibitors,lubricants for water-based drilling fluids and emulsifying surfactantsfor invert emulsion oil-based drilling fluids. U.S. Pat. No. 4,108,779describes the use of (apparently calcium salts of) oleic acid dimers tocontrol the viscosity of water-in-oil spacer fluids. U.S. Pat. No.4,607,700 and U.S. Pat. No. 5,193,618 describe the use of a dimer of analphaolefin sulphonate surfactant to form a foam steam drive injectionfluid for hydrocarbon discovery.

DEFINITIONS

The terms “carbo”, “carbyl”, “hydrocarbon”and “hydrocarbyl”, when usedherein, pertain to compounds and/or groups which have only carbon andhydrogen atoms.

The term “saturated” when used herein, pertains to compounds and/orgroups which do not have any carbon-carbon double bonds or carbon-carbontriple bonds.

The term “unsaturated” when used herein, pertains to compounds and/orgroups which have at least one carbon-carbon double bond orcarbon-carbon triple bond.

The term “aliphatic”, when used herein, pertains to compounds and/orgroups which are linear or branched, but not cyclic (also known as“acyclic” or “open-chain” groups).

The term “cyclic”, when used herein, pertains to compounds and/or groupswhich have one ring, or two or more rings (e.g., Spiro, fused, bridged).Compounds with one ring may be referred to as “monocyclic” or“mononuclear” whereas compounds with two or more rings may be referredto as “polycyclic” or “polynuclear”.

The term “ring”, when used herein, pertains to a closed ring of from 3to 10 covalently linked atoms, more preferably 3 to 8 covalently linkedatoms.

The term “aromatic ring”, when used herein, pertains to a closed ring offrom 3 to 10 covalently linked atoms, more preferably 5 to 8 covalentlylinked atoms, which ring is aromatic.

The term “heterocyclic ring”, when used herein, pertains to a closedring of from 3 to 10 covalently linked atoms, more preferably 3 to 8covalently linked atoms, wherein at least one of the ring atoms is amultivalent ring heteroatom, for example, nitrogen, phosphorus, silicon,oxygen, and sulfur, though more commonly nitrogen, oxygen, and sulfur.

The term “alicyclic”, when used herein, pertains to compounds and/orgroups which have one ring, or two or more rings (e.g., spiro, fused,bridged), wherein said ring(s) are not aromatic.

The term “aromatic”, when used herein, pertains to compounds and/orgroups which have one ring, or two or more rings (e.g., fused), whereinsaid ring(s) are aromatic.

The term “heterocyclic”, when used herein, pertains to cyclic compoundsand/or groups which have one heterocyclic ring, or two or moreheterocyclic rings (e.g., Spiro, fused, bridged), wherein said ring(s)may be alicyclic or aromatic.

By an “oligomeric” or “oligomer” surfactant we mean that the structureof the surfactant is based on from two to eight (and preferably two tofive) linked surfactant monomer subunits, each monomer subunit having apolar head group (which may be a cationic, anionic or zwitterionicgroup) and a C₁₀-C₅₀ organic (i.e. aliphatic, alicyclic or aromatic)tail group bonded at a terminal carbon atom thereof to the head group.Preferably the C₁₀-C₅₀ organic tail group is a hydrocarbyl tail group.The monomer subunits are linked in the oligomer either headgroup-to-head group or tail group-to-tail group. When they are linkedhead group-to-head group, the oligomer has distinct tail groupscorresponding to the tail groups of the monomer subunits and asuper-head group formed from the plural head groups of the monomersubunits. When they are linked tail group-to-tail group, the oligomerhas distinct head groups corresponding to the head groups of the monomersubunits and a super-tail group formed from the plural tail groups ofthe monomer subunits.

Although the oligomer is defined above in relation to achemically-corresponding monomer subunit, in practice the oligomersurfactant may not necessarily be synthesised from that monomer. Forexample, a synthesis route may be adopted in which monomer subunits arefirst oligomerised and the head groups are then changed to those of thedesired oligomer surfactant. That is the head groups of the monomersubunits used in practice to form the oligomer may be different from thehead groups of the monomer subunits to which the oligomer chemicallycorresponds. In another example, if the tail groups of the monomersactually used to form the oligomer are unsaturated, the oligomerisationprocess may involve the partial or total hydrogenation of those groups,particularly if the tail groups are linked in the oligomer.

Furthermore the tail groups of the monomer units actually used to formthe oligomer may be aliphatic, but if the monomer units are linked inthe oligomer tail group-to-tail group, the links formed between the tailgroups in the super-tail group may be aliphatic, alicyclic or aromatic.

By a “viscoelastic” fluid we mean that the elastic (or storage) modulusG′ of the fluid is equal to or greater than the loss modulus G″ asmeasured using an oscillatory shear rheometer (such as a Bohlin CVO 50)at a frequency of 1 Hz and at 20° C. The measurement of these moduli isdescribed in An Introduction to Rheology, by H. A. Barnes, J. F. Hutton,and K. Walters, Elsevier, Amsterdam (1997).

By “straight chain” we mean a chain of consecutively linked atoms, allof which or the majority of which are carbon atoms. Side chains maybranch from the straight chain, but the number of atoms in the straightchain does not include the number of atoms in any such side chains.

SUMMARY OF THE INVENTION

We have found that oligomer surfactants can be used to form viscoelasticwellbore treatment fluids with distinctive and useful properties.

In a first aspect the present invention provides a viscoelastic wellboretreatment fluid comprising an effective amount of an oligomericsurfactant for controlling the viscoelasticity of the fluid. Preferably,the viscoelasticity of the fluid is maintained up to at least 50° C.

We have found that surfactants of this type are particularly suitablefor use as wellbore thickening agents. The surfactants form aqueousviscoelastic solutions via micellar aggregation but have a reducedtendency, compared with monomeric surfactants, to locate at theoil/water interface. That is, the oligomeric surfactant molecules areless surface active and so do not reduce the oil/water interfacialenergy to the same extent. This helps to inhibit the formation ofoil/water emulsions and promotes oil and water separation.

Compared with monomeric surfactants, oligomeric surfactants also tend tohave higher viscosities at higher temperatures. So the useful workingtemperatures of wellbore treatment fluids based on viscoelasticsufactants can be increased.

Another advantage of these surfactants is that they generally providehigher viscosities per unit weight of surfactant than the correspondingmonomers. So less surfactant may be needed for a particular task whichreduces e.g. materials, transportation and storage costs. In oneembodiment the treatment fluid comprises less than 10 percent by weightof oligomeric surfactant. It may comprise less than five and preferablyless than three percent by weight of oligomeric surfactant. In generalwe have found that, compared to the corresponding monomer surfactant,approximately half the amount by weight of an oligomer surfactant isneeded to produce a treatment fluid with similar performancecharacteristics.

Preferably the viscosity of the treatment fluid is breakable on contactwith hydrocarbons, such as kerosene, so that the viscosity at 20° C. isreduced by at least 80%. Although breaking may be demonstrated bycontacting equal volumes of treatment fluid and oil, the skilled personknows that solutions based on viscoelastic surfactants are generallybreakable by relatively small amounts of oil, breaking being a complexprocess typically involving molecular rearrangement and larger scalefluid fingering processes. H. Hoffmann and G. Ebert in “Surfactants,micelles and Fascinating Phenomena”, Angew. Chem. Int. Ed. Engl., 27,902-912 (1988) provide a discussion of breaking phenomena.

The structure of the oligomeric surfactant may be based on linkedsurfactant monomer subunits, each monomer subunit having the formula(R₁—X)_(p)Z_(m) or R₁—Y; wherein X is a charged head group, Z is acounterion, p and m are integers which ensure that the surfactantmonomer is charge neutral, Y is a zwitterionic polar headgroup (such as—N⁺(CH₃)₂—CH₃—COO⁻ or —N⁺(CH₃)₂—CH₃—OSO₃ ⁻), and R₁ is a C₁₀-C₅₀ organic(preferably hydrocarbyl and/or aliphatic) tail group comprising aC₁₀-C₂₅ (preferably C₁₅-C₂₄) straight chain bonded at a terminal atomthereof to respectively X or Y.

The oligomeric surfactant may be formed in situ from the correspondingoligomeric acid precursor. The organic tail group may comprise only thestraight chain. The straight chain may be a hydrocarbyl chain. In oneembodiment the monomer straight chain is unsaturated. Preferably theoligomer is a dimer or a trimer.

Preferably X is a carboxylate (—COO⁻), sulphate (—OSO₃ ⁻), sulphonate(—SO₃ ⁻), phosphate (—OPO₃ ²⁻), or a phosphonate (—PO₃ ²⁻) chargedgroup. Z may be an alkali metal counterion. For the avoidance of doubt,it is hereby mentioned that when X is a carboxylate group the carbonatom of the carboxylate group is not counted with the carbon atoms ofthe organic tail group. The surfactant monomer may be a salt of oleicacid.

Alternatively X may be a quaternary ammonium (—NR₂R₃R₄ ⁺) charged group;R₂, R₃ and R₄ being C₁-C₆ aliphatic groups, or one of R₂, R₃ and R₄being a C₁-C₆ aliphatic group and the others of R₂, R₃ and R₄ forming afive- or six-member heterocylic ring with the nitrogen atom. In oneembodiment the monomer units are linked tail group-to-tail group, andpreferably straight chain-to-straight chain, in the oligomer.Preferably, R₂, R₃ and R₄ are each and independently a —CH₃, —CH₂CH₃,—CH₂CH₂CH₃, —CH(CH₃)₂, —CH₂OH, —CH₂CH₂OH, —CH₂CH₂CH₂OH, —CH₂CH(OH)CH₃,—CH(OH)CH₂CH₃, —CH(CH₂OH)CH₃ or —C(CH₃)₂OH group. R₁ may be an erucylgroup or an oleyl group. Z may be a halide anion such as Cl⁻ or Br⁻, oran organic anion with a molecular weight of less than 300 such assalicylate or octyl sulphate.

In one embodiment the complete oligomer can be defined by the formula(R₁R₂R₃N⁺—(—R₅—N⁺(R₁R₄)—)_(n)—R₅—N⁺R₁R₂R₃)._((n+2))Z⁻ where n=0, 1, 2 or3, and R₅ comprises a C₁-C₁₂ aliphatic group (and is preferably ahydrocarbyl chain and more preferably an unbranched C₁, C₂, C₃, C₄, C₅or C₆ aliphatic chain) or a C₅-C₁₂ aromatic or alicyclic group.

The treatment fluid may further comprise an effective amount of amonomeric surfactant for controlling the viscoelasticity of the fluid.Generally, monomeric surfactants generate maximum viscosities atrelatively low temperatures, while oligomeric surfactants generatemaximum viscosities at relatively high temperatures. So by adjusting therelative amounts of the oligomeric and monomeric surfactants therheological behaviour of the fluid can be predetermined or controlled.The relative amounts of the oligomeric and monomeric surfactants may beadjusted so that the viscosity of the treatment fluid, as measured usinga steady shear rheometer (such as a Bohlin CVO 50) at a shear rate of100 s⁻¹, is at least 10 cP for all temperatures in the range 80 to 260°F. (26.5 to 126.5° C.), and is preferably at least 50 cP for alltemperatures in the range 120 to 260° F. (49 to 126.5° C.).

In various embodiments the treatment fluid is, or is used as,respectively a fracturing fluid, a water shut-off treatment fluid, or aselective acidizing fluid.

In a particular embodiment the treatment fluid is a scale dissolverfluid for dissolving scale in a subterranean hydrocarbon-bearingformation, the fluid further comprising an effective amount of a scaledissolver formulation, whereby, in use, formation hydrocarbons act onthe surfactant to reduce the viscosity of the fluid so that the fluidselectively invades a hydrocarbon-bearing zone of the formation. Inparticular, the surfactant of the scale dissolver fluid may comprise asalt of an oligomer of oleic acid as described in earlier application GB0019380.5.

In use, the fluid of this embodiment is injected into the subterraneanformation in a relatively viscous state. If the injected fluid contactsa watered-out zone of the formation the viscous nature of the fluidremains essentially unaltered and, to a significant extent, the fluid isprevented from entering the watered-out zone, i.e. the fluid locally haslimited injectivity. Conversely, if the fluid contacts ahydrocarbon-bearing zone of the formation the viscosity is locallysignificantly reduced and the fluid is able to penetrate thehydrocarbon-bearing zone.

Therefore, the difference in viscosity of the fluid when in contact withhydrocarbons and water advantageously allows a selective placement ofthe scale treatment, and as a result scale may be preferentially removedfrom hydrocarbon-bearing zones. This can lead to a stimulation ofhydrocarbon production without a substantial increase in the water cutof produced fluids.

Preferably the scale dissolver formulation activates the production ofviscoelasticity by the surfactant. In this way it may not be necessaryto add additional agents, such as KCl brine, to activate the productionof viscoelasticity. However, the use of such additional agents is notexcluded by the present invention. The scale dissolver formulation maycomprise any acid or alkaline solution that dissolves minerals and otherwellbore deposits (including organic deposits). Desirably the scaledissolver formulation comprises an aqueous solution of at least one ofan alkali metal carbonate, alkali metal hydroxide, EDTA and an alkalimetal salt of EDTA. The alkali metal may be potassium. Alternatively thescale dissolver formulation may comprise a mineral acid, such as HCl.

A second aspect of the present invention provides a method of dissolvingscale in a subterranean formation with at least one hydrocarbon-bearingzone, the method including pumping the scale dissolver fluid of theparticular embodiment of the first aspect of the invention through awellbore and into the subterranean formation, the viscosity of the scaledissolver fluid being reduced by formation hydrocarbons so that thefluid selectively invades the hydrocarbon-bearing zone of the well todissolve scale in the hydrocarbon-bearing zone.

A third aspect of the present invention provides a method of injecting ascale dissolver fluid into a subterranean formation with at least onehydrocarbon-bearing zone, the method including the step of pumping thescale dissolver fluid of the particular embodiment of the first aspectof the invention through a wellbore and into the subterranean formation.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described withreference to the following drawings in which:

FIGS. 1 a-e show typical chemical structures of dimeric components ofoleic acid oligomer mixtures,

FIG. 2 compares the temperature dependencies of the viscosities (at ashear rate of 100 s⁻¹) of 2.25, 3 and 4.5 weight percent aqueoussolutions of three potassium oleate dimers (E1016, E1018 and U1009),each solution containing 8 weight percent potassium chloride,

FIG. 3 shows the dependence, at five temperatures in the range 25-80°C., of the viscosity on shear rate of a solution containing 3 weightpercent of the potassium salt of U1009 and 6 weight percent of potassiumchloride,

FIG. 4 shows the temperature dependence of the storage (G′) and loss(G″) moduli of the solution of FIG. 3 measured at an oscillatoryfrequency of 1 Hz,

FIG. 5 a shows an inverted bottle containing a gel formed from thesolution of FIG. 3, and FIG. 5 b shows a bottle containing the same gelafter shaking with an equal volume of kerosene,

FIG. 6 a shows a solution containing 3 weight percent of oleic aciddimer E1018 and 8 weight percent potassium chloride to which solutionhas been added an equal quantity of a 500 ppm (0.013 molar) aqueoussolution of calcium ions, and FIG. 6 b shows the same mixture ofsolutions but with the F1018 dimer replaced with E1016,

FIG. 7 shows a graph comparing the rheology of two scale dissolverfluids comprising oleic acid oligomers at 60° C.,

FIG. 8 shows a graph comparing the injectivities into oil andwater-saturated cores of a scale dissolver fluid,

FIG. 9 shows schematically the steps involved in deploying a scaledissolver fluid of the present invention,

FIGS. 10 a and b show respectively dimers ofN-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride andN-oleyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride,

FIG. 11 shows the reaction used to synthesiseN-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride,

FIG. 12 shows a graph of viscosity against temperature for a 3 weightpercent solution of dimericN-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride in de-ionizedwater,

FIG. 13 shows graphs of viscosity against temperature for solutionscontaining 3 weight percent of dimericN-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride andrespectively 0.2, 0.5 and 0.7 weight percent of NH₄Cl, and solutionscontaining respectively 2 and 4 weight percent of monomericN-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride and 3 weightpercent of NH₄Cl,

FIG. 14 shows graphs of viscosity against temperature for solutionscontaining 4 weight percent of monomericN-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride and 3 weightpercent of NH₄Cl; 1 weight percent of dimericN-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride and 3 weightpercent of monomeric N-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammoniumchloride; and 2 weight percent of dimericN-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride, 1 weightpercent of monomeric N-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammoniumchloride and 1 weight percent of NH₄Cl,

FIG. 15 shows viscosity versus temperature profiles for a solutioncontaining 1 wt % oleyl-dimer and 3 wt % erucyl-monomer and a solutioncontaining 4 wt % erucyl-monomer, and

FIG. 16 shows a synthesis route for forming a quaternary ammonium dimerin which the monomer units are linked at their organic tail groups.

DETAILED DESCRIPTION

Viscoelastic solutions of both anionic and cationic oligomericsurfactants were investigated.

A controlled stress rheometer (Bohlin model type CVO-50) was used tomeasure the Theological properties of the solutions. Using a concentriccylinders (Couette) geometry (inner radius of the outer cylinder,R_(i)=1.375 cm, outer radius of the inner cylinder, R_(o)=1.25 cm, andinner cylinder length=3.78 cm), corresponding to the geometry of GermanDIN standard 53019, the viscosity of each solution was measured atseveral applied shear stresses within a specified range. The typicalrange of shear stress was 0.5-40 Pa corresponding to a shear rate rangeof 0.005 to 1000s⁻¹. Measurements were made at increasing and thendecreasing shear rate. Typically, the complete set of measurementsconsisted of 40 viscosity measurements, each taken after a delay time of10 seconds at constant shear stress and shear rate.

For the particular geometry of the rheometer, the shear rate wascalculated as:

${\overset{.}{\gamma} = {\frac{{{RPM} \cdot 2}\;\pi}{60}\frac{{2 \cdot R_{i}^{2}}R_{o}^{2}}{\left( \frac{R_{i} + R_{o}}{2} \right)^{2}\left( {R_{o}^{2} - R_{i}^{2}} \right)}}},$where RPM is the rotational speed (in revolutions per minute) of theinner cylinder. The viscosity was then obtained for each measurement bydividing the measured stress by the calculated shear rate.Oligomeric Anionic Surfactants

The oligomerisation of oleic acid generally leads to the production ofcomplex mixtures of dimeric and trimeric products. Commerciallyavailable oligomers, such as the Empol™ series of dimers and trimersfrom Henkel Corporations Chemical Group (4900 Este Avenue-Bldg 53,Cincinnati, Ohio 45232, USA) are suitable for putting the presentinvention into operation. Alternative suppliers of suitable mixtures aree.g. Uniqema (PO Box 90, Wilton Center, Middleborough, Cleveland TS908JE, UK), Union Camp (Vigo Lane, Chester-le-Street. Co. Durham DH3 2RB,UK) and Expo Chemical Company Inc. (12602 Manorwood, Cypress (Houston),Tex. 77429, USA). FIGS. 1 a-e show typical chemical structures ofdimeric components of these mixtures. Clearly the components havedifferent degrees of hydrogenation.

Dimer anionic surfactants were generated from the potassium salts ofcommercially available oleic acid dimer mixtures (although forconvenience the mixtures will be referred to as if they were individualdimers). In the absence of electrolyte (such as potassium salts),solutions of the potassium oleate oligomers containing up to 6 weightpercent surfactant were found to form low viscosity liquids. However, inthe presence of potassium salts, such as potassium chloride, thesolutions become viscoelastic and readily formed strong gels.

The potassium oleate oligomer surfactants were made directly in aqueoussolution by the addition of the liquid oligomer acid to a solution ofpotassium hydroxide. The extent of the reaction was monitored bymeasuring pH, substantially fully converted potassium oleate oligomersolutions having a pH in the range 8-9.

FIG. 2 compares the measured viscosities (at a shear rate of 100 s⁻¹) of2.25, 3 and 4.5 weight percent aqueous solutions of three potassiumoleate dimers, each solution containing 8 weight percent potassiumchloride. The labels E1016 and E1018 refer to the trade names of theoleic acid dimers, Empol™ 1016 and Empol™ 1018, produced by the HenkelCorporation, while U1009 refers to a hydrogenated oleic acid dimerproduced by Uniqema. E1016 contains a relatively high amount ofaliphatic super-tail group (i.e. non-head group) structures, while E1018has a larger amount of alicyclic and aromatic super-tail groupstructures.

The solutions of the potassium salt of the hydrogenated oleic acid dimerU1009 were significantly more viscous than the corresponding solutionsformed from E1016 and E1018. This is believed to be due to the higherdegree of saturation of the U1009 super-tail group.

FIG. 3 shows the dependence of the measured viscosity of a solution ofthe potassium salt of U1009 (3 weight percent) with potassium chloride(6 weight percent) on shear rate at five temperatures in the range25-80° C. At ambient temperature and at low shear rates the viscosity ofthe solution was in excess of 100 poise, although the viscositydecreased sharply with increasing shear rate. At higher temperatures theviscosity was significantly less dependent on shear rate and approachedNewtonian behaviour.

FIG. 4 shows the temperature dependence of the storage (G′) and loss(G″) moduli of the same solution measured at an oscillatory frequency of1 Hz. When the temperature was below about 50° C., G′>G″ and thesolution was viscoelastic. Above this temperature the loss modulusdominated and the solution became predominantly viscous. The temperatureat which the solution lost its viscoelasticity corresponded to that atwhich the viscosity lost its marked dependence on shear rate.

The viscosities of the solutions of the potassium oleate dimers, gelledby the addition of potassium salts, were reduced on contact withhydrocarbons. FIG. 5 a shows a bottle containing the solution of thepotassium salt of U1009 (3 weight percent) with potassium chloride (6weight percent). The surfactant solution formed a rigid gel as evidencedby the retention of the solution at the base of the bottle even when thebottle was inverted. FIG. 5 b shows a bottle containing the same gelafter shaking with an equal volume of kerosene (dyed red to aidcontrast). The viscoelasticity of the solution was destroyed by contactwith the hydrocarbon and the kerosene floated on the surfactantsolution. The surfactant solution and the kerosene did not form a stableemulsion. Similar tests were performed using both aliphatic and aromatichydrocarbons, including pure aromatic hydrocarbons such as toluene andxylene. The tests showed that the lack of stable emulsion formationbetween the oligomer solutions and the hydrocarbons is a characteristicproperty of these surfactants.

Solutions of the potassium oleate dimers were observed to respond to theaddition of soluble calcium ions in different ways. This is significantbecause Ca²⁺ ions are often found in mixed and formation water. FIG. 6 bshows a copious fine white precipitate which developed when a solutionconsisting of 3 weight percent of oleic acid dimer E1016 and 8 weightpercent potassium chloride was mixed with an equal quantity of a 500 ppm(0.013 molar) aqueous solution of calcium ions. In contrast, when thecorresponding experiment was performed on dimer E1018 (FIG. 6 a) only afew large pieces of white precipitate developed and the solutionmaintained its clarity. The hydrocarbon chains of E1018 are moreunsaturated than those of the E1016, which indicates that higher degreesof saturation may be advantageous when the mixed or formation watercontains significant levels of dissolved calcium. In contrast, thecationic surfactants discussed below were relatively unaffected bydissolved calcium.

Dimer acids were also used to form scale dissolver fluids. A scaledissolver fluid of the present invention has enhanced rheologicalperformance which allows it to dissolve scales preferentially inhydrocarbon-bearing matrices of subterranean formations. To asignificant extent this performance is due to the ability of the fluidto vary its viscosity depending on whether it is in contact with wateror hydrocarbons. In contrast, conventional scale dissolver fluids removescale deposits indiscriminately from hydrocarbon and water-bearing zonesalike.

If the scale dissolver fluid is considered as a combination of aconventional scale dissolver fluid and the surfactant, the viscosity ofthe gel can be reduced to substantially that of the conventional fluidwhen the gel comes into contact with hydrocarbons, making the scaledissolver formulation of the fluid readily injectable intohydrocarbon-bearing matrices. However, when the gel contacts water itremains highly viscous (and therefore not easily injectable), anyreduction in viscosity being essentially due to dilution. Effectivelythe highly viscous gel acts as a diverting agent and allows a highproportion of the scale dissolver formulation to be placed inhydrocarbon zones.

Scale dissolver fluid example 1: EDTA (13 g), potassium hydroxide (11.25g) and potassium carbonate (2.25 g) were dissolved in water (70.5 g),and E1016 oleic acid dimer (3 g) was then added and the mixture stirreduntil it became a homogeneous gel.

Scale dissolver fluid example 2: EDTA (8.66 g), potassium hydroxide (7.5g) and potassium carbonate (1.5 g) were dissolved in water (79 g), andEmpol™ 1043 oleic acid trimer (3 g) was then added and the mixturestirred until it became a homogeneous gel.

The viscosities of the gels of examples 1 and 2 were measured at 60° C.over a range of shear rates. The results of these measurements are shownin FIG. 7. Both gels exhibited Newtonian rheology over a surprisinglywide range of shear rates. Advantageously, therefore, the injectivity ofthe gels into subterranean matrices should not be affected by changes inshear rate which may occur during the placement process.

A 150 cP gel based on the formulation of example 1 was injected into anoil-saturated core and a water-saturated core by forcing the gel down asupply line which branched into two parallel lines leading to the twocores. Both cores were of Bentheimer sandstone and had equal total porevolumes. By measuring the relative amounts of gel entering the two coresat a given supply pressure or for a given volume of supplied gel, therelative injectivities of the gel through the two cores was determined.

Injection profiles of the gel into the two cores with the fluid andcores maintained at a temperature of 60° C. are shown in FIG. 8. Thepermeability of the water-saturated core was 1.6 darcies while that ofthe oil-saturated core was 1.4 darcies; both cores had a porosity of22%. The profiles demonstrate that the volume of gel entering theoil-saturated core is approximately 50% greater than that entering thewater-saturated core. The preference of the gel to enter theoil-saturated core is maintained even after a large number of porevolumes was passed through the two cores. The viscosity of the effluentfrom the oil-saturated core was significantly lower than that of theinjected gel throughout the duration of the experiment and demonstratedthat the surfactant gel was continually mix with oil. In contrast, theviscosity of the effluent from the water-saturated core was similar tothat of the injected gel. Higher viscosity fluids enhance this contrastand fluids can be developed that only enter oil-bearing zones, theviscosity being too high for injection into the water-bearing zones.

FIG. 9 shows schematically the steps involved in the deployment of ascale dissolver fluid of the present invention.

Oligomeric Cationic Surfactants

A dimer of N-erucyl-N,N-bis(2-hydroxyethyl)-N-methylanmonium chloride(FIG. 10 a) was synthesized by linking the head groups via a C₄ bridge.

FIG. 11 shows the reaction used to synthesise the dimer. To a mixture ofbis(hydroxyethyl)erucyl amine (50.00 g, 123.2 mmol) and1,4-dibromobutane (12.97 g, 60.08 mmol) was added 100 g of ethanol assolvent. The reaction was carried out under reflux and was monitored bytitration and Thin Layer Chromatograph (TLC). TLC plates, under UVlight, showed the formation of a single product and the disappearance ofthe starting material. The reaction was stopped when both the acidtitration and TLC results indicated completion of the reaction (after 24hours). The solvent was removed under vacuum. A light yellow waxy solidwas collected as the product.

The monomeric surfactant,N-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride, does notform a gel in de-ionized water, and requires at least 0.2 mol/L chlorideto induce gelation and the range 0.3 to 0.5 mol/L chloride to reach themaximum viscosity. In contrast, a solution containing 3 weight percentof the corresponding dimer surfactant in de-ionized water wasviscoelastic, the viscosity remaining relatively constant over thetemperature range of 80 to 160° F. (26.5 to 71° C.), as shown in FIG.12.

For a solution containing 3 weight percent of the dimer ofN-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride and 0.2-0.5weight percent NH₄Cl, a maximum in the viscosity measured was observedat 215cF (101.5° C.) (FIG. 13). When the concentration of NH₄Cl wasraised to 0.7 weight percent, the maximum shifted to 220° F. (104.5°C.). The dimer formulations had a viscosity in excess of 50 cP in thetemperature range 190 to 230° F. (88 to 110° C.), whereas the viscosityof the 2 wt % and 4 wt % monomer formulations fell below 50 cP at 175and 190° F. (79.5 and 88° C.), respectively (FIG. 13).

In general, the dimer-based formulations containing NH₄Cl had lowerviscosities at the lower temperatures compared with the formulationsbased on the monomer. This indicates that treatment fluids based on suchdimer surfactants should be more manageable on surface (i.e. at the wellhead).

Improved high temperature rheology was obtained by blending the dimerwith the monomer. Some typical results are presented in FIG. 14. Ingeneral, solutions containing mixtures of dimer and monomer showedreduced viscosities at the lower temperatures and enhanced viscosity attemperatures above 150° F. (65.5° C.), compared with solutions basedonly on the monomer. For example, a solution containing 2 weight percentdimer, 1 weight percent monomer and 1 weight percent NH₄Cl solution hada low viscosity (measured at 100 s⁻¹) at 80° F. (26.5° C.), butmaintained a viscosity >50 cP (at 100 s⁻¹) in the broad temperaturerange 110 to 260° F. (43.5 to 126.5° C.). By comparison, the viscosityof a solution containing 4 weight percent monomer and 3 weight ammoniumchloride fell below 50 cP when the temperature was increased abovearound 190° F. (88° C.). The comparison clearly demonstrates theadvantage of using the dimeric surfactant in combination with themonomeric surfactant for fracturing or other applications in hightemperature environments.

We also found that the improved high temperature viscosifyingperformance of the dimer/monomer blend was achieved using a lower totalconcentration of surfactant and lower inorganic brine (NH₄Cl)concentration as compared to the monomer-only formulation.

The dimer of N-oleyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride(FIG. 10 b), was also synthesised by linking the head groups with a C₄bridge. This dimeric surfactant is a white solid at room temperature,and is poorly soluble in water at room temperature. However, thesolubility of this surfactant increases with increasing temperature.

It was discovered that theN-oleyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride diner could besolubilised by blending it with theN-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride monomer. Theviscosity versus temperature profiles for a solution containing 1 wt %oleyl-dimer and 3 wt % erucyl-monomer is compared to a solutioncontaining 4 wt % erucyl-monomer in FIG. 15. The dimer/monomer blendshowed reduced viscosity at the lower temperatures up to around 140° F.(60° C.) as compared to the solution of erucyl-monomer. Again thisindicates that a treatment fluid based on such a dimer/monomer blendshould be relatively manageable under surface conditions.

Also the viscosity of the dimer/monomer blend at temperatures aboveabout 140° F. (60° C.) was greater than that of the erucyl-monomersolution, which again indicates that a treatment fluid based on thedimer/monomer blend is applicable under a greater range of down holetemperatures, particularly, in this case, for the range 140 to 210° F.(60 to 99° C.), compared to a fluid based on a solution containing onlythe monomer.

As with the anionic surfactant solutions, the viscoelasticity of boththe oleyl- and erucyl-based oligomeric surfactant solutions wasdestroyed by the addition of hydrocarbons. This was also true for thedimer/monomer blends whose viscosity versus temperature profiles areshown in FIGS. 14 and 15.

Also tests like those discussed above in relation to FIGS. 5 a and bshowed that the cationic dimer surfactants and dimer/monomer surfactantblends had a reduced tendency to form stable emulsions with hydrocarbonsas compared with the corresponding cationic monomeric surfactants.

FIG. 16 shows a synthesis route for forming an alternative form ofoligomeric cationic surfactant in which the monomer units are linkedtail group-to-tail group instead of head group-to-head group. R₁, R₂ andR₃ are e.g. methyl groups. In the particular synthesis shown thestarting point is oleic acid which is then dimerised to form oleic aciddimer. In fact, as discussed above, oleic acid dimers are commerciallyavailable products (e.g. E1016, E1018 and U1009), so it is actually moreconvenient to start the synthesis with the dimer. The dimer is nextconverted in two steps to the corresponding quaternary ammonium dimer.This is an example of an oligomer which has a chemically-correspondingmonomer repeat unit (N-oleyl-N,N,N-tris(methyl) ammonium chloride) whichis different from the monomer (oleic acid) used to form the oligomer inpractice.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

1. A method of treating a subterranean formation which comprises atleast one hydrocarbon-bearing zone, comprising providing an oligomericsurfactant with a structure comprising from 2 to 8 linked surfactantmonomer subunits, each monomer subunit having a formula (R₁ X)_(p)Z_(m),where X is a carboxylate charged group, R₁ is a C₁₀-C₅₀ organic tailgroup comprising a C₁₀-C₂₅ straight chain bonded at a terminal atomthereof to X, Z is an alkali metal counterion, and p and m are equal to1 so that the surfactant monomer subunit is charge neutral, making aviscoelastic treatment fluid which is an aqueous micellar solutioncontaining a thickening amount of said oligomeric surfactant and wheremicellar aggregation of surfactant makes said solution viscoelastic,wherein the wellbore treatment fluid further comprises at least one ofan alkali metal carbonate, an alkali metal hydroxide, EDTA, an alkalimetal salt of EDTA and a mineral acid, so that the fluid functions todissolve scale, and pumping said viscoelastic treatment fluid through awellbore and into the subterranean formation whereupon contact withhydrocarbons within the formation dissipates the viscosity of saidtreatment fluid and causes said treatment fluid to selectively invade ahydrocarbon-bearing zone of the formation.
 2. A method according toclaim 1 wherein R₁ is a C₁₅- C₂₄ straight chain.
 3. A method accordingto claim 1, wherein the oligomer is formed from two to five monomersubunits.
 4. A method according to claim 1, wherein the oligomer is adimer or a trimer.
 5. A method according to claim 1 wherein saidtreatment fluid further comprises a monomeric surfactant.
 6. A methodaccording to claim 5 wherein the relative amounts of the oligomeric andmonomeric surfactants are adjusted so that the viscosity of thetreatment fluid, as measured using a steady shear rheometer at a shearrate of 100 s⁻¹, is at least 10 cP for all temperatures in the range 80to 260° F. (26.5 to 126.5° C.).
 7. A method according to claim 1 whereinthe thickening amount of said oligomeric surfactant in said treatmentfluid is less than 10 percent by weight.